1. Field of the Invention
The present invention relates to a method of and apparatus for recording an image on a photosensitive material which are available in laser plotters, process scanners laser plotters and other image scan recorders, and more particularly to an improvement in recording the image by scanning a photosensitive plane in parallel by means of a plurality of photobeams or multibeams.
2. Description of the Prior Arts
In a laser plotter for recording a wiring pattern on a photosensitive film to produce a master film for printed circuit boards, for example, the photosensitive film is scanned with laser beams which are modulated by image signals expressing the wiring pattern. FIG. 19 shows a schematic construction of a laser plotter, which comprises an image data generator 110, a data converter 120 and an image recorder 130. Graphic figures representing a wiring pattern are produced with a CAD system, and vector data VD expressing the contour lines of the graphic figures are generated in the image data generator 110. The vector data VD are then transmitted to the data converter 120 to be converted into dot image data. The image recorder 130 has a laser unit 131 generating a plurality of laser beams or multibeams LB, and the multibeams LB are modulated into ON/OFF binary multibeams in accordance with the dot image data for each pixel.
A photosensitive film F is wounded around a recording drum 132 which is rotated in a direction .phi.. When the modulated multibeams LB are supplied onto the photosensitive film F while moving the multibeams LB in a direction X, the photosensitive film F is scanned with the modulated multibeams LB through a combination of main scannings by the rotation .phi. and a subscanning by the linear movement in the direction X. Since a plurality of scanning lines are scanned in parallel by means of the multibeams LB, respective line images corresponding to the plurality of scanning lines are recorded in parallel at a strip region on the photosensitive film F. The whole of the wiring pattern is obtained on the photosensitive film F strip by strip by repeating the main scanning while moving the multibeams LB in the subscanning direction X. The image recording using the multibeams LB is superior to that using only a single photobeam in recording speed of the whole image.
FIG. 20 is a diagram showing a photospot array which is formed on the photosensitive film F by the multibeams LB. The photospot array will be visible only after the photosensitive film F is developed, and consisits of five photospots BS being aligned in the subscanning direction X which is perpendicular to the main scanning direction Y. Although the image recording using the multibeams LB is preferred for a high speed recording, two problems are caused when the photospots BS are aligned at an alignment pitch being equal to the diameter of each photospot, as shown in FIG. 20. One of them is that respective line images on the neighboring scanning lines are not smoothly connected since the intensity distribution of each photospot drastically falls at the edge of the photospot in accordance with a gaussian distribution function, and exposure at the edge portion is often unsufficient for effective recording. The other problem is that the quality of the recorded image is hardly increased because rough steps appear at a boundary between an exposed region and an unexposed region, as shown in FIG. 21.
In order to solve the problems, the following improvements have been developed.
(1) In the first improvement, the laser beams are so arranged that the photospots BS partially overlap each other, as shown in FIG. 22. To avoid interference between overlapped coherent laser beams, the respective polarizations of neighboring beams are orthogonalized to each other. The overlap of the photospots compensate the lack of exposure in the edge of the photospots, and the boundary between the exposed and unexposed regions becomes fine steps as shown in FIG. 23.
(2) In the second improvement, partially overlapped photobeams are supplied on to the photosensitive film F through respective light paths whose difference is larger than the coherent length of the laser beams.
(3) In the third improvement, the photospots are spatially staggered in the main scanning direction Y, as shown in FIG. 24. Respective timings in modulating the laser beams are alternately changed for compensating the spatial stagger of the photospots BS. Respective traces of the neighboring photospots are also overlapped with each other, and the recorded image shown in FIG. 23 is obtained similarly to the improvements (1) and (2).
(4) In the fourth improvement, respective laser beams are individually generated by means of a plurality of laser oscillators. Respective traces of the neighboring beams are partially overlapped with each other similarly to the improvements (1) through (3).
Although the two problems in the alignment of FIG. 20 are solved by either of the improvements, respective new problems are caused. That is, the first improvement has a disadvantage in that additional optical means for rotating polarization by 90 degrees, a quarter wavelength plate, for example, is required and an optical system becomes complicated. A disadvantage of the second improvement is that at least one of the light paths must be very long whereby the optical system often goes out of its regular arrangement when the optical system is mechanically vibrated by an external force, even if the degree of vibration is small. The staggered arrangement of the photospots in the third improvement requires a circuit for adjusting the modulation timings of respective laser beams, so that the circuit structure in the image recorder becomes complicated. Furthermore, the fourth improvement causes a problem in that the cost for fabricating the image recorder and the complexity of the structure are increased because two or more laser oscillators must be provided. Therefore, an improvement for solving the two problems without these new problems is being required in the field of image recording.
Furthermore, employment of the partially overlapped laser beams cases another problem that the recording speed is decreased due to increase of scanning lines in density. This problem cannot be solved as long as the lase beams are partially overlapped with each other, and it is unreasonable to employ the overlapped beams when image quality in high resolution is not required.